The display capacity of liquid crystal display devices using a liquid crystal panel has been recently increased.
In a simple matrix structured liquid crystal display device employing a multiplex driving system, the contrast drops or the response speed reduces as a time-shared system is further developed. Accordingly, if the liquid crystal display device has about 200 scanning lines, it is difficult to obtain a sufficient contrast.
Accordingly, there has been employed an active matrix system liquid crystal panel having switching elements in each picture-element to remove such drawbacks.
In the active matrix system liquid crystal panel, there are two types: one is a three terminal system employing thin film transistors (hereinafter referred to as "TFT") serving as switching elements and the other is a two terminal system employing thin film diodes (hereinafter referred to as "TFD") serving as nonlinear resistors. The two terminal system is superior to the three terminal system since the former is simple in structure and easier to produce.
A diode type, a varsity type, and an MIM (Metal-Insulator-Metal) type have been developed as the two terminal system.
The structure of a liquid crystal display device employing conventional thin film diodes will be now described with reference to FIG. 20 which is a plan view showing a part thereof and FIG. 21 which is a cross sectional view taken along the line A--A in FIG. 20.
The liquid crystal display device comprises, as shown in FIG. 21, a first substrate 1 and a second substrate 2 respectively made of a transparent material such as glass, wherein the first and second substrates 1 and 2 oppose each other with a certain gap via a spacer 3 and a liquid crystal 4 is filled in between the first and second substrates 1 and 2.
A lower electrode 5 and a picture-element electrode (display electrode) 6 are disposed on the first substrate 1 in a matrix shape as shown in FIG. 20, and an anodic oxidation layer 7 as a nonlinear resistor layer is formed on the lower electrode 5. Further, a pair of upper electrodes 8 and 9 are disposed on the anodic oxidation layer 7 so as to overlap each other.
A thin film diode (TFD) 11 as a first nonlinear resistor element is structured by the lower electrode 5, the anodic oxidation layer 7 and the upper electrode 8, and a TFD 12 as a second nonlinear resistor element is structured by the lower electrode 5, the anodic oxidation layer 7 and the upper electrode 9.
The upper electrode 8 of the first TFD 11 is connected with a signal electrode 13 (FIG. 20) which applies a signal from the outside to the TFD 11, and the upper electrode 9 of the second TFD 12 is connected to the picture-element electrode 6.
A switching element part is structured by the first TFD 11 and the second TFD 12, and an electric path is formed in the order of: "the signal electrode 13.fwdarw. the upper electrode 8.fwdarw. the anodic oxidation layer 7.fwdarw. the lower electrode 5.fwdarw. the anodic oxidation layer 7.fwdarw. the upper electrode 9.fwdarw. the picture-element electrode 6".
On the other hand, a black matrix 14 is provided on the surface of the second substrate 2 opposing the first substrate 1 at an entire region thereof as hatched in FIG. 20 for preventing leaking of light from a gap between each picture-element electrode 6 disposed on the first substrate 1. That is, the black matrix 14 is provided on a non-display part for intercepting light.
Further, an opposing electrode 15 formed as a scanning electrode is disposed on the second substrate 2 while opposing the picture-element electrode 6 as shown in FIG. 21 and it is arranged in a belt shape as shown in FIG. 20 via an insulating film 16 so as not to contact the black matrix 14 and be short circuited.
In FIG. 21, the lower electrode 5, the upper electrodes 8 and 9, the picture-element electrode 6 and the signal electrode 13 respectively on the first substrate 1 are shown by broken lines, and the anodic oxidation layer 7 is omitted in the illustration, and the black- matrix 14 and the opposing electrode 15 under the second substrate 2 are shown by solid lines.
Orientation films 17 and 18 are provided at opposing surfaces of the first and second substrates 1 and 2 as processing layers for regularly arranging molecules of the liquid crystal 4. Deflector plates, not shown, are provided at outside surfaces of the first substrate 1 and the second substrate 2, and they are irradiated by back light by a surface light source, not shown, from the direction of an arrow 19.
The picture-element electrode 6 is arranged to overlap the opposing electrode 15 via the liquid crystal 4, and it becomes the picture-element part of a liquid crystal display panel, wherein the liquid crystal display device performs a given image display owing to the change of the light transmittance caused by the change of the orienting state of the liquid crystal 4 on the region where the black matrix 14 is not formed on the picture-element electrode 6.
FIG. 22 is an equivalent circuit of the liquid crystal display device, wherein the signal electrode 13 and the opposing electrode (scanning electrode) 15 are formed in a matrix shape, and the first TFD 11 and the second TFD 12 are arranged with each other with a back-to-back connection between the signal electrode 13 and the opposing electrode 15, and there is formed a switching circuit which conducts from the picture-element electrode 6 to the opposing electrode 15 via the capacitor of the liquid crystal 4.
When a voltage having a value exceeding a threshold value is selectively applied between the signal electrode 13 and the opposing electrode 15, an ON current flows in the switching circuit between the signal electrode 13 and the opposing electrode 15 so as to turn the orientation of the liquid crystal 4 between the picture-element electrode 6 and the opposing electrode 15, thereby allowing light to pass through the liquid crystal 4.
Whereupon, characteristics of the thin film diode at plus and minus sides can be made symmetric when two TFDs 11 and 12 are used with a back-to-back connection although the thin film diode (TFD) frequently demonstrates asymmetric voltage-current characteristics depending on the polarity of an applied voltage.
However, the switching circuit can be structured with one TFD for every picture-element, and an equivalent circuit of the liquid crystal display device in such a case becomes like the one as illustrated in FIG. 23. Denoted by 10 is a TFD like the first and second TFDs 11 and 12, and it structures a switching circuit between each signal electrode 13 and each opposing electrode (scanning electrode) 15 wherein the TFD 10 is in series with the capacitor of the liquid crystal 4 between the picture-element electrode 6 and the opposing electrode 15.
A method of fabricating a thin film diode (TFD) by a conventional treatment when producing the liquid crystal display device shown in FIGS. 20 to 22 is explained with reference to a plan view in FIG. 24 and cross sectional views in FIGS. 25 to 29 respectively showing each fabricating step.
First of all, a lower electrode layer material 21 made of tantalum is formed on the entire surface of the first substrate 1 (hereinafter referred to merely as "substrate") made of glass by a sputtering treatment shown in FIG. 25.
Thereafter, a photoresist is formed on the entire surface of the lower electrode layer material 21 by a spin coating method, then an exposing and developing treatment is performed using a given photo mask, thereby forming a first photoresist 22 which was subject to a patterning treatment in the shapes of lower electrode layers 21a and 21b as shown in FIG. 26.
In the aforementioned processing steps, there is a likelihood that dust particle 31 may be formed on the lower electrode layer material 21 owing to the sticking of dust immediately after the formation of the lower electrode layer material 21 made of tantalum, or the sticking of dust on the first photoresist 22 or contamination of the photo mask.
Thereafter, the lower electrode layer material 21 is subject to a patterning treatment using the first photoresist 22 as an etching mask by a dry etching technique, thereby forming the lower electrode layers 21a and 21b as shown in FIG. 26.
In the etching treatment for forming the lower electrode layers 21a and 21b, the dust particle 31 becomes an etching mask, thereby forming the lower electrode layer 21a having a pattern shape which is larger than the prescribed dimensions. The lower electrode layer 21b having a pattern shape which is larger than the prescribed dimensions is formed depending on the sticking position of the dust particle 31.
The plane pattern shapes of the lower electrode layer 21a forming the lower electrode 5 of the thin film diode (TFD), the lower electrode layer 21b forming the lower layer of the signal electrode 13 and a lower electrode layer 21c formed in surplus are respectively shown by solid and broken lines in FIG. 24, and the region where they finally remain is hatched.
Thereafter, as shown in FIG. 26, the lower electrode layers 21a to 21c are subject to an anodic oxidation treatment at the surfaces thereof, and the anodic oxidation layers 7 serving as the nonlinear resistor layer are formed on each surface of the lower electrode layers 21a to 21c with a film thickness of 35 nm.
Further, as shown in FIG. 27, a transparent electrode film 24, e.g., made of indium tin oxide (ITO), as a transparent conductive film is formed on the entire surfaces of the anodic oxidation layers 7 with the film thickness ranging from 100 to 200 nm by a sputtering treatment.
Then, a photoresist is formed on the entire surface of the transparent electrode film 24 by the spin coating method, and an exposing and developing treatment is performed using a given photo mask, thereby forming a second photoresist 25 having upper layer patterns of the two upper electrodes of the thin film diodes, the picture-element electrode and the signal electrode.
Thereafter, the transparent electrode film 24 is subject to etching using the second photoresist 25 as an etching mask, thereby forming the upper layer patterns of the upper electrodes 8 and 9 of the first and second TFDs, the picture-element electrode 6 and the signal electrode 13 as shown in FIG. 28.
In FIG. 24, the plane pattern shapes of the picture-element electrodes 6 are shown by a one-dotted and chain line and the plane pattern shape of the signal electrode 13 is shown by a two-dotted and chain line. The picture-element electrodes 6 and signal electrode 13 are also formed on the lower electrode layers 21b and 21c.
A driving IC (semiconductor device) connection part 29 for each line is formed on the tip end of each signal electrode 13.
A protection film 26 made of tantalum pentoxide is formed on the entire surface in a thickness ranging from 100 to 200 nm by a sputtering treatment as shown in FIG. 28.
Thereafter, a photoresist is formed on the entire surface of the protection film 26, and an exposing and developing treatment is performed using a given photo mask, thereby forming a third photoresist 27 having an opening pattern 27a.
The opening pattern 27a of the third, photoresist 27 is formed at positions corresponding to a connection part (common electric part) 21d between the lower electrode layers 21a and 21b, and to the driving IC connection part 29 in FIG. 24, whereby the opening portions 26a and 26b are defined in the protection film 26 as shown by a two-dotted and chain line and dots or points.
That is, the connection part 21d of the lower electrode layers and the protection film 26 on the driving IC connection part 29 are subject to etching by a dry etching technique using the third photoresist 27 as an etching mask, thereby forming the protection film 26 having the opening portions 26a and 26b (the opening portion 26b is shown in FIG. 24 alone).
The anodic oxidation layer 7 and the connection part 21d of the lower electrode layer made of tantalum are sequentially subject to etching through the opening 26a.
As a result, the lower electrode layer 21a is separated from the lower electrode layer 21b of the signal electrode 13 as shown in FIG. 24, thereby forming the island-shaped lower electrodes 5 for the first TFD 11 and the second TFD 12.
The protection film 26 is formed for preventing short circuits from occurring between the signal electrode 13 and the picture-element electrode 6 respectively formed on the first substrate 1 on which the TFDs 11 and 12 are provided and the opposing electrodes (scanning electrodes) on the second substrate 2 opposing the signal electrode 13 and the picture-element electrode 6. In FIG. 24, the protection film 26 is not illustrated for convenience of illustration thereof, but only the opening portions 26a and 26b are illustrated.
However, according to such fabricating steps, an anomalous thin film diode part 30 is formed by the lower electrode layer 21c protruded from the lower electrode layer 21b of the signal electrode 13 by the dust particle 31 other than the first TFD 11 connected with the signal electrode 13 and the second TFD 12 connected with the picture-element electrode 6 as shown in the plan view in FIG. 24 and the cross-sectional view in FIG. 29.
When the anomalous thin film diode part 30 is formed in such a manner, the ratio of capacitance between the TFDs 11 and 12 and the liquid crystal is changed, thereby changing a threshold value of the liquid crystal driving voltage. Accordingly, even if a given voltage is applied between the signal electrodes 13 and the opposing electrodes 15 as shown in FIG. 22, an ON-current does not flow, thereby forming picture-elements in which the oriented states of the liquid crystal 4 are not changed, leading to the occurrence of the problem that the liquid crystal display device has a line defect.
Further, as shown in FIG. 24, there is a likelihood of occurrence of defects such as a short circuit 32 between the picture-element electrodes where the picture-element electrodes 6 are short-circuited, a short circuit 33 between the signal electrode and the picture-element electrode where the signal electrode 13 and the picture-element electrode 6 are short-circuited, or a short circuit 34 between signal electrodes where the signal electrodes 13 are short-circuited.
The defects of these short circuits occur owing to the remenant of etching of the lower electrode layer material 21 made of tantalum or transparent electrode film 24, and there is a problem that the liquid crystal display device will have a line defect owing to such occurrence of short circuits, thereby deteriorating the display quality.
The problem in the liquid crystal display device using such a thin film diode (TFD) as switching elements will occur likewise in a liquid crystal display device using thin film transistors (TFT) as switching elements.
This problem will be now described with reference to FIG. 30. FIG. 30 is a plan view showing a part of an active matrix substrate constituting a liquid crystal display device using conventional thin film transistors.
The active matrix substrate of this liquid crystal display device corresponds to the first substrate of the conventional liquid crystal display device, and comprises a substrate 1 made of transparent glass, and a gate electrode G made of the same material and a scanning electrode 41 respectively provided on the substrate 1.
An anodic oxidation layer is formed on the surfaces of the gate electrode G and scanning electrode 41, and an insulating coating film is provided on the anodic oxidation layer, thereby constituting a gate insulating film together with the anodic oxidation layer, which is however not illustrated.
A source region 47s, a drain region 47d and a semiconductor layer 47 forming a central channel region are respectively provided on the gate insulating film, and a channel stop layer 48 is provided on the semiconductor layer 47. A picture-element electrode 50 comprising a transparent electrode film is provided.
A source electrode S which is continuous with a signal electrode 42 via an ohmic contact layer (not shown) and a drain electrode D which is connected with the picture-element electrode 50 are respectively provided on the channel stop layer 48, thereby structuring a thin film transistor 40.
Further, an accumulation capacitor C having a metal-insulating film-metal! structure is structured by a region opposing the thin film transistor 40 of the picture-element electrode 50, a large width part 41a formed in the scanning electrode 41 as shown by broken lines in the same figure, and a gate insulating film, not shown, provided between the region opposing the thin film transistor 40 and the large width part 41a.
Driving IC connection parts 44a and 44b for mounting driving ICs for driving the liquid crystal display device are respectively provided at end parts of the scanning electrodes 41 and the signal electrodes 42.
Even in such an arrangement, there is a likelihood of occurrence of defects of short circuits 53a to 53e between electrodes, namely, between the picture-element electrode 50 in the picture-elements and the signal electrode 42, between the picture-element electrode 50 and the scanning electrode 41, between the signal electrodes 42 serving as a signal path to the driving ICs for driving the liquid crystal display device, between the scanning electrodes 41 and between the scanning electrode 41 and the signal electrode 42, which are respectively caused by dust particle generated in the exposing and developing step of the photoresist like the case where the thin film diode is fabricated on the substrate.
In the liquid crystal display device where one picture-element area is miniaturized, the dimensions of gaps between respective electrodes become small, and hence such defects of short circuits frequently occur.
The defects of short circuits such as the short circuit between the picture-elements and that between the electrodes in the liquid crystal display device using such thin film diodes or thin film transistors as switching elements are caused by the sticking of dust during the process of patterning of a photoresist, etc. Most of the dust is generated with the manufacturing apparatus or human beings acting as the dust generation sources, and it is very difficult to eliminate such generation of dust. With the defects of such short circuits, a display defect frequently occurs in the liquid crystal display device, leading to the deterioration of the display quality. This also causes poor yield when producing the liquid crystal display device.
Whereupon, active matrix type liquid crystal display devices have recently required a large screen with high quality, and they have been widely applied to a viewfinder or a view terminal employing a liquid crystal panel, a projection TV, or a navigation TV, and hence they further required high yield with a high precision pattern.
The present invention solves the aforementioned problems, and it is an object of the invention to provide a liquid crystal display device using thin film diodes or thin film transistors as switching elements capable of improving display quality thereof and capable of producing a liquid crystal display device having a high display quality with high yield.